1. Field of the Invention
The present invention relates to a bias circuit for an avalanche photodiode.
The invention is particularly concerned with a bias circuit for making an avalanche photodiode operate stably.
2. Description of the Prior Art
An avalanche photodiode is employed as an element receiving a faint optical signal in fields of optical communication and optical measurement.
The avalanche photodiode is applied reverse voltage which is slightly less than its breakdown voltage, so that receiving an optical signal, optical multiplication (3 to 100) is obtainable by the avalanche phemonenon. The larger the optical multiplication is, the slighter optical signal is receivable. Many improvements have accordingly been done in order to obtain stably the higher optical multiplication.
The voltage which is slightly less than the breakdown voltage of the avalanche photodiode is required in order to obtain large optical multiplication. However, the reverse current of the avalanche photodiode increases abruptly with a little increase of the reverse voltage in adjacencies of the breakdown voltage so that optical multiplication increases, too. Accordingly, the larger optical multiplication is required, the slighter fluctuation of the reverse voltage the same is chamged with. In such a case, the avalanche photodiode may be damaged by the abrupt increase of the reverse current.
Such changes of the optical multiplication and the reverse current can also be caused by variations of the environmental temperature, even if the applied reverse voltage is constant.
There is shown in FIG. 1 a prior art bias circuit to deal with such a problem.
In FIG. 1, 11 indicates an avalanche photodiode of which cathode is connected to a base of a common emitter type transistor of the first stage in a signal amplifier 13. When an optical signal 12 is applied to the avalanche photodiode 11, an electrical signal multiplied with the avalanche phenomenon is obtained at the cathode of the avalanche photodiode 11. The electrical signal is amplified by the signal amplifier 13 connected to the cathode of the avalanche photodiode 11.
Numeral 17 identifies an operational amplifier, 24 and 25 denote transistors, 41 to 46 designate resistors and 53 represents a potentiometer.
The transistor 24, the resistors 43 to 46 and the potentiometer 53 construct a constant current source. The collector current of the transistor 24 flows to a negative power source -V.sub.D through the resistors 42 and 41.
The avalanche photodiode 11 is made of silicon, for instance, which is applied with reverse voltage of 130 volts being slightly less than the breakdown voltage. The resistance of the resistor 42 is 10 megohms, for instance, and the collector current of the transistor 24 is about 13 microamperes, so that the negative feedback loop consisting of the operational amplifier 17, the transistor 25 and the resistor 42 operates to keep the anode voltage of the avalanche photodiode 11-130 volts.
In case the optical multiplication is constant, the fluctuations of the breakdown voltage of the avalanche photodiode 11 caused by fluctuations of the environmental temperature are about 0.1% of the breakdown voltage a centigrade degree (130 mV/a centigrade degree).
The fluctuations of the voltage V.sub.EB between the emitter and the base of the transistor 24 are -2.5 millivolts a centigrade degree, too.
In case the environmental temperature rises, the voltage V.sub.EB of the transistor 24 decreases, so that the collector current increases and the voltage drop across the resistor 42 increases, too. The voltage at the point in which resistor 42 and the operational amplifier 17 are connected is fixed to zero volts with the operation of the negative feedback loop consisting of the operational amplifier 17, the transistor 25 and the resistor 42. Therefore, the voltage at the point connected with the resistor 42 and the avalanche photodiode 11 increases toward minus. In such a manner, the optical multiplication is kept constant by increasing slightly the reverse voltage of the avalanche photodiode 11 of which optical multiplication decreases a little by the environmental temperature rising up.
A dividing ratio of the emitter current of the transistor 24 through the resistor 44 can be adjusted with the potentiometer 53. By such means, a rate of change of the voltage between the emitter and the base of the transistor 24 is adjustable.
The voltage in adjacencies of the breakdown voltage of the avalanche photodiode 11 is obtained by the voltage drop across the resistor 42 through which the collector current of the transistor 24 flows.
The voltage drop across the resistor 42 must be a large voltage (e.g. 130 V). It is therefore required that resistance of the resistor 42 is large or current (the collector current of the transistor 24) through the resistor 42 is large.
In an optical time domain reflectometer (OTDR) employing necessarily such an avalanche photodiode for measuring disconnection and other problems of optical fibers, for instance, low power consumption is generally required because the OTDR is frequently used with battery operation in outdoor measurement of fiber cables during a long time. The more the collector current of the transistor 24 is increased, the more the transistor 24 itself generates heat. Therefore, the temperature characteristic of the voltage between emitter and the base of the transistor 24 is undesirably influenced by not only the environmental temperature but the heat generated by the transistor 24 itself. If the collector current of the transistor 24 is 13 microamperes and the resistance of the resistor 42 is 10 megohms, the reverse voltage of 130 volts is obtainable across the resistor 42.
If a resistor 42 having high resistance (e.g. 10 megohms) is employed, its temperature coefficient is not better than the same having low resistance.
In the prior art, there have, therefore, been problems that the temperature compensation of the avalanche photodiode has not been obtained enough by the temperature coefficient of the resistor 42.
However, a resistor having good temperature coefficient has not been employed as the resistor 42, because of increasing the generation of heat in the transistor 24 and the power comsumption as above-mentioned.